Police in Washington today announced charges against a dozen Turkish security agents who were involved in a violent altercation during the Turkey's president visit to the US last month.

In a statement, the police department said a total of 18 people face charges, and that two had already been arrested.

Twelve of the number are Turkish security agents, who have outstanding arrest warrants for their alleged part in the clash, which left nine protesters injured.

The action is likely to exacerbate what has become a major irritant in US-Turkish ties.

Relations were severely strained even before the May 16 clash, which happened as President Recep Tayyip Erdogan arrived at the Turkish ambassador's residence in DC after a White House meeting with President Donald Trump.

The Nato allies are still at odds over a US decision to arm Syrian Kurdish rebels fighting the Islamic State of Iraq and the Levant (Isil) group in Syria, whom they consider terrorists.

Washington police officials confirmed that two Americans have already been charged over the altercation.

Sinan Narin, of Virginia was charged with felony and misdemeanor assault, and Eyup Yildirim, who can be seen on video repeatedly kicking a protester, was charged with two felony counts and one misdemeanour assault count.

The police statement did not say if the men were supporters of Mr Erdogan, part of his security detail or protesters.

Coming almost a month after the episode, the charges are the most significant retaliatory step taken to date by American authorities, who have fumed privately and publicly over what they see as a highly offensive attack on free speech.

Mr Erdogan's security detail returned with him to Turkey after his visit, so it is unclear if any will face legal repercussions in the US. However, they could end up being threatened with arrest if they return.

If any are still in the country, they could be expelled if Turkey refuses to waive diplomatic immunity.

Video of the protest showed security guards and some Erdogan supporters attacking a small group of protesters with their fists and feet.

Men in dark suits and others were recorded repeatedly kicking one woman as she lay curled on the pavement.

"The charges filed against 12 Turkish security officials sent a clear message to the United States that it does not tolerate individuals who use intimidation and violence to stifle freedom of speech and legitimate political expression," said Rex Tillerson, the US Secretary of State.

"The State Department will continue to work with law enforcement and with relevant legal authorities in the case. When an outcome is reached, the department will determine if any additional steps will need to be taken."